The Brigadier summons the Doctor back to Earth to investigate mysterious
goings-on around Loch Ness in Scotland. The Doctor, Sarah Jane and Harry
discover that the Loch Ness Monster is no myth -- in fact, it is really
the Skarasen, a cybernetic reptile used as a servant by shape-shifting
aliens known as the Zygons. The Zygons are paving the way for an invasion
of Earth, and have already used their powers to infiltrate the local
authorities.

Production

As Robert Holmes settled fully into his role as Doctor Who's script
editor and finished work on stories initiated by the outgoing production
team, he began to cast about for new writers to commission for the
programme. One candidate was an acquaintance of his named Robert Banks
Stewart. Stewart had begun his career as a playwright before moving into
television; amongst his many credits were The Avengers, Danger
Man, The Saint and The Sweeney. Stewart had spent some
time in Australia, where he had produced Intrigue. He had also been
approached to contribute to Doctor Who during the programme's
formative stages in 1963, though nothing had come of this.

A native of Scotland, Stewart was keen to set a Doctor Who
adventure in his home country, and suggested to Holmes that the legendary
Loch Ness Monster would make an ideal basis for a story, given that so few
details existed about the nature of the mythical monster. Belief that
some kind of creature -- possibly prehistoric in origin -- dwells in the
loch may date back as much as 1500 years. It gained new popularity when a
road passing by Loch Ness was constructed in 1933, with interest in the
tale reaching a global scale in the 1960s. On March 12th, 1974, Holmes
commissioned Stewart to prepare a six-part storyline under the title
“Loch Ness”, with the request for scripts following on the
27th.

Robert Banks Stewart wrote the Doctor, Sarah and Harry in
the vein of the protagonists of The
Avengers

Stewart wrote his scripts over the remainder of the year. In the process,
the length of the serial was curtailed from six installments to four.
Unfamiliar with the style of Doctor Who, Stewart wrote the Doctor,
Sarah and Harry in the vein of the protagonists of The Avengers,
and had to be guided by Holmes in refining these elements.

Stewart originally placed a far greater emphasis on the Skarasen and the
loch; the initial cliffhanger for episode two, for instance, involved the
monster attacking the Doctor as he rows across Loch Ness, rather than
traversing Tullock (originally Tulloch) Moor. Holmes felt that the Zygons
were more interesting characters, however, especially since they could
interact with the main cast, and asked Stewart to keep the focus of the
serial firmly on them. As the scripts were rewritten, Sister White became
Sister Lamont, the name of Stansgate House was changed to Stansbridge
House, and Claymore Oil was rechristened Hibernian Oil.

During the early part of 1975, the adventure variously went by the titles
“The Loch Ness Monster”, “The Secret Of The Loch”,
“The Secret Of Loch Ness”, and “Secret Of Loch
Ness”. By now, Serial 4F had been assigned to director Douglas
Camfield, making his return to Doctor Who after a five-year
absence. Camfield had last helmed Inferno,
during which he had collapsed when the stress of production aggravated a
heart murmur. He had been on a self-enforced sabbatical from the programme
ever since, but was lured back by the concept of Stewart's story.

At the same time, several production decisions were being made which would
have a significant effect on “Secret Of Loch Ness”. Throughout
1974, buzz had been building about Gerry and Sylvia Anderson's new
science-fiction series Space: 1999, which was being developed for
ITV, the commercial channel which was the BBC's principal rival. As 1975
dawned, it was apparent that Space: 1999 was set for a September
debut. Since 1970, Doctor Who had been part of the BBC's winter
programming slate, debuting in late December or early January. Concern now
arose that if Doctor Who's thirteenth season started three months
after Space: 1999 launched, viewing figures might be
compromised.

Philip Hinchcliffe later admitted that Robert Holmes was
right to believe that Harry was a valuable character

Consequently, in January it was decided that Season Thirteen would be
brought forward to a late summer/early fall start, as had been the case
throughout most of the Sixties. This meant that the thirteenth recording
block, which normally wouldn't begin until September, would also have to
start earlier. To facilitate this, Season Twelve was trimmed from
twenty-six episodes to only twenty -- the shortest in Doctor Who
history ot that point. “Secret Of Loch Ness”, scheduled to be
the final serial of the twelfth recording block and originally intended to
be the culmination of the twelfth season, would instead be held over to
start Season Thirteen. Furthermore, the usual break between recording
blocks would be eliminated entirely, with the twelfth and thirteenth
blocks running together to essentially form one marathon production
slate.

Meanwhile, Hinchcliffe also determined that Harry Sullivan should be
written out of Doctor Who at the end of “Secret Of Loch
Ness”. The character had been created at a time when it was thought
that the Fourth Doctor might be played by an elderly actor who would be
unable to participate in action sequences; with the casting of Tom Baker
in the lead role, Hinchcliffe felt that Harry was now redundant. Holmes
disagreed with the producer, believing that Harry was a valuable and
distinctive ingredient of the series but nonetheless had Stewart amend his
scripts appropriately; Hinchcliffe would later admit that Holmes' views
had been correct.

The exit of Harry Sullivan also removed one more link between the series
and the Earthbound UNIT format which had propelled it through the early
Seventies. Hinchcliffe wanted to set more adventures on alien worlds, and
actors Nicholas Courtney and John Levene, who had been contracted for
“Secret Of Loch Ness” to once again play the Brigadier and
Benton, became aware that their irregular appearances in Doctor Who
might soon cease altogether. Meeting with Hinchcliffe in the BBC bar,
Courtney suggested that the Brigadier should be killed off, exiting the
series in a blaze of glory. Hinchcliffe turned down the idea, however,
feeling that it would be an inappropriate fate for a character who had
been so vital to the programme.

Nicholas Courtney suggested to Philip Hinchcliffe that
Brigadier Lethbridge-Stewart should be killed off

Work on “Secret Of Loch Ness” began with location filming in
West Sussex; sadly, the budget did not stretch to allow cast and crew to
actually travel to Scotland. On the first day, March 17th, beach scenes
were recorded at Climping Beach in Climping, while the arrival of the
TARDIS took place on Ambersham Common in South Ambersham. Work continued
on the Common the next day for sequences on Tullock Moor, while both the
Zygon spaceship's landing area and the hospital exterior were actually
Hall Aggregates Quarry, Storrington. The 19th was spent entirely at
Ambersham Common, dealing with more material set both on the Moor and in
the woods, as well the dematerialisation of the TARDIS.

On March 20th, the venue shifted to Charlton, with filming occurring at
both the Fox Inn and a barn. The day after, Furnace Pond in Crabtree posed
as Loch Ness. This should have marked the end of recording in West Sussex,
but poor weather earlier in the week forced the allocation of two extra
days. March 24th saw a return to Hall Aggregates Quarry for another
hospital exterior shot, while the 25th was again spent at Ambersham
Common, once more pressed into service as Tullock Moor. The same day,
Elisabeth Sladen was contracted for twenty-two episodes of the thirteen
recording block; Tom Baker's services had already been secured for all
twenty-six episodes on the 21st.

On March 26th, footage of Millbank Tower in London was captured for use in
the climactic Skarasen sequence. By now, both stop-motion animation and
puppetry had been employed to bring the Skarasen to life. Unfortunately,
this material proved to be of inferior quality, and Camfield decided to
use as little of it as possible.

The first studio session then occurred in BBC Television Centre Studio 3.
Monday, April 7th involved the taping of episode one, with the second
episode following on Tuesday the 8th. Around this time, Hinchcliffe
elected to change the serial's title, as he feared that emphasising the
Loch Ness Monster was misrepresenting the story's contents. The adventure
became “The Zygons” and finally Terror Of The Zygons.
Recording shifted to slightly later in the week for the second studio
block, which took place in TC4. Part three was completed on Tuesday, April
22nd, along with the lone Fargill Castle scene from part four. The
remainder of the last installment was then taped on Wednesday the
23rd.

Ian Marter and Tom Baker wrote the screenplay for an
unmade feature film called Doctor Who Meets
Scratchman

The completion of Terror Of The Zygons brought Doctor Who's
twelfth production block to a close, although work immediately proceeded
onto the next set of episodes. This also meant that Ian Marter's time as a
regular Doctor Who artiste had come to an end. Marter would
continue to maintain a connection to Doctor Who, however: after
returning for one more appearance as Harry shortly thereafter, in The Android Invasion, Marter went on to write
several novelisations of Doctor Who stories for Target Books. He
also authored an original Harry Sullivan novel, Harry Sullivan's
War, for Target's short-lived The Companions Of Doctor Who
range. He twice made unsuccessful bids to write for the TV series itself
(once in 1980, and later with a submission entitled either “Strange
Encounter” or “Volovox” for Season Twenty-Three), and
with Tom Baker cowrote the screenplay for an unmade Doctor Who
feature film called Doctor Who Meets Scratchman. Marter also
continued to act on stage and television, earning credits on programmes
such as Bergerac and The Return Of Sherlock Holmes.
Unfortunately, Marter died of a diabetic condition on October 30th,
1986.

Inadvertently, Terror Of The Zygons also proved to be Nicholas
Courtney's final Doctor Who appearance for many years, when
scheduling issues prevented him from returning for The Android Invasion. Nonetheless, Courtney
would eventually reprise the role of Lethbridge Stewart in the Eighties,
in Mawdryn Undead, The
Five Doctors, and Battlefield.
Following the demise of the original Doctor Who series, Courtney
played the Brigadier in the thirtieth-anniversary special Dimensions In Time, two radio plays starring
Jon Pertwee as the Third Doctor (The Paradise Of Death and The
Ghosts Of N-Space), and Death Comes To Time, the first original
Doctor Who webcast. Courtney also lent his voice to several
Doctor Who audio adventures from Big Finish Productions, including
The Spectre Of Lanyon Moor, Minuet In Hell, Zagreus,
Sympathy For The Devil, and installments of the UNIT
series. He finally returned to play the Brigadier on television in 2008,
in the Sarah Jane Adventures serial Enemy Of The Bane. Away from
Doctor Who, Courtney continued to work prolifically, with his
television appearances encompassing shows like All Creatures Great
And Small, Yes, Prime Minister and The Bill. Courtney
died on February 22nd, 2011 after a battle with cancer.